Five gunmen sprayed bullets into crowds of worshippers waiting for the start of a Sunday service at a church in Biu Town in Borno state, and a suicide bomber detonated explosives outside another church in Jos, 260 miles away.

At least four people died and more than 40 people were injured, including several who remained in critical condition last night.

The two incidents are the latest attacks on church congregations in Nigeria's majority-Muslim north, where an armed Islamist group, Boko Haram, has vowed to force Christians to leave.

There were no immediate claims of responsibility for Sunday's strikes, but both bore hallmarks of Boko Haram.

"The gunmen came to the premises of the church and started firing at people outside before going into the main building to carry on their killings," said Hamidu Wakawa, who was at the Biu Town church.

At least one woman was killed and three other people injured as the men opened fire on the church. They escaped before bystanders could catch them, witnesses said.

In Jos, rescue workers were still clearing the rubble from the Christ Chosen Church last night. The force of the suicide bomber's explosives completely destroyed the church, said Abuh Emmanuel, a local police spokesman.

The bomber and two churchgoers died and police said 41 other people were treated for their injuries.

Boko Haram has been blamed for hundreds of killings in bomb or gun attacks over the past two years, and it is increasingly choosing targets over an ever wider area of northern Nigeria.

The group began as a local militia targeting people who broke strict Islamic regulations such as drinking alcohol But it recently linked with al-Qaeda's franchise in West Africa, al-Qaeda in the Islamic Maghreb, and appears to have taken on far more ambitious aims, including ridding northern Nigeria of Christians.

It claimed responsibility for a suicide bombing of UN headquarters in Abuja, Nigeria's capital, in August last year that killed at least 25 people.

It carried out a suicide attack on the Abuja office of one of the country's most prominent newspapers and claimed responsibility for a bomb attack at a church near the capital on Christmas Day which killed at least 44 people.

Boko Haram's leader, Abubakar Shekau, frequently justifies attacks on Christians as revenge for the killings of Muslims in Nigeria's volatile "Middle Belt", where Jos is located and where the largely Christian south and mostly Muslim north meet.

Boko Haram, which has linked up with other Islamist groups in the region including al-Qaeda's north African wing, has become the biggest security threat in Nigeria.